Method for monitoring broadcast signals at alternative frequencies and gain control unit

ABSTRACT

Many broadcast stations transmit their programs via a variety of different frequencies. The invention relates to monitoring alternative frequencies during the reception of a broadcast signal at a present frequency. Whenever the broadcast signal at an alternative frequency is checked, the receiver&#39;s gain is instantaneously switched from a present gain value corresponding to the present frequency to a second gain value corresponding to the alternative frequency. The second gain value is adapted to the supposed signal strength of the broadcast signal at the alternative frequency.

The invention relates to a gain control unit, to a receiver and to amethod for monitoring broadcast signals at alternative frequenciesduring the reception of a broadcast signal at a present frequency.

Many radio stations transmit their radio programs via a variety ofdifferent broadcast frequencies. When receiving a certain radio programat a present frequency, a receiver may at the same time monitoralternative frequencies, in order to compare the signal strength and thesignal quality at said alternative frequencies to the receptionconditions of the present frequency. In case the signal received at thepresent frequency is impaired by distortions, the receiver may switch toa different one of the variety of frequencies on which the respectiveradio program is transmitted. Thus, the multitude of availablefrequencies guarantees a certain quality level of the received signal.

Switching from one received frequency to another frequency requires toreadjust the gain of the receiver circuitry. Depending on the respectivetype of the automatic gain control loop used in the receiver, the gainadjustment might take quite long. Depending on the design of the gaincontrol loop, fast gain adjustments using short time constants lead toinstabilities of the loop.

It is therefore an object of the invention to speed up the monitoring ofbroadcast signals at alternative frequencies.

The object of the invention is solved by a method for monitoringbroadcast signals at alternative frequencies according to claim 1 and bya receiver according to claim 12. Preferred embodiments thereof arerespectively defined in the following dependent sub-claims. A computerprogram product according to the present invention is defined in claim14.

According to the invention, broadcast signals at alternative frequenciesare monitored during the reception of a broadcast signal at a presentfrequency. Whenever the broadcast signal at an alternative frequency ischecked, the receiver's gain is instantaneously switched from a presentgain value corresponding to said present frequency to a second gainvalue corresponding to an alternative frequency, whereby the second gainvalue is adapted to the supposed signal strength of the broadcast signalat said alternative frequency.

In prior art solutions, the receiver's gain has been adjusted by meansof the receiver's closed-loop control circuit. Especially in case thegain of the RF amplifier is varied, it takes quite a while until thecontrol loop has settled to the correct gain value. During the settlingtime, the signal quality of the broadcast signal received at analternative frequency cannot be evaluated.

According to the invention, whenever the broadcast signal is switched toan alternative frequency, a corresponding gain value is written to theautomatic gain control circuit. Thus, the correct gain value isavailable right from the beginning. The settling time (in the order of10-20 msec, depending on the AGC design) is replaced by a setting timeof less than 1 msec, and therefore, only a small period of time isrequired for checking a broadcast signal at a certain alternativefrequency. A lot of different alternative frequencies can be monitoredduring the reception of a broadcast signal at the present frequency. Inorder to optimise the reception conditions, the receiver selects thefrequency where the signal quality is best.

According to a preferred embodiment of the invention, it is determinedwhether the program transmitted via the broadcast signal at saidalternative frequency is the same as the program transmitted. Thelistener has selected the program he wants to listen to. Therefore,switching to an alternative frequency may only be performed if theprogram transmitted via said alternative frequency is the same as theprogram transmitted via the present frequency.

Preferably, the signal strength of the broadcast signal received at thealternative frequency is compared to the signal strength of thebroadcast signal received at the present frequency. In case it turns outthat the signal strength of the broadcast signal at the alternativefrequency is higher than the signal strength at the present frequency,is makes sense to switch to said alternative frequency, because thereception conditions at said alternative frequency are likely to besuperior to the present reception conditions. In case the signalstrength of the alternative frequency is worse than the signal strengthat the present frequency, it is better to stay at the present frequency.

Preferably, in case the signal strength of the broadcast signal at thealternative frequency surpasses the signal strength of the signal at thepresent frequency by a predefined amount, and in case the programstransmitted at both frequencies are identical, the received frequency isswitched from the present frequency to the alternative frequency. Beforeswitching is performed, two prerequisites have to be fulfilled: theaudio program transmitted at the alternative frequency has to be thesame as the present audio program, and the signal quality at thealternative frequency channel has to be superior to the present signalquality.

According to a preferred embodiment of the invention, both the broadcastsignal received at said present frequency and the broadcast signalreceived at said alternative frequency are broadcast signals accordingto the DRM standard. The standard DRM (Digital Radio Mondiale) is thedigital equivalent of an analogue broadcast signal in the medium waverange of radio transmission.

Preferably, alternative frequencies are monitored during time slots ofstatic data symbol transmission, whereby during a first time slot, thereceiver's gain control circuit settles to said second gain value, andwhereby during a second time slot of static data symbol transmission,the receiver's gain is instantaneously switched to said second gainvalue. According to the standard DRM, the transmission of a data streamcorresponding to the respective audio program is periodicallyinterrupted by time slots of static data symbol transmission. Duringthese time slots, a predefined sequence of bits is transmitted, and forthis reason, these time slots can be used for monitoring alternativefrequencies. Especially in case of a slowly reacting gain controlcircuit, the length of a single time slot is too short for adjusting thereceiver's gain to the new broadcast signal, for detecting whether thesame program is transmitted, and for determining the signal strength atsaid alternative signal. Therefore, a first time slot is spent fordetermining the correct gain value for the signal at said alternativefrequency. At the end of the first time slot, the receiver's gaincontrol circuit has settled to the correct gain value, and said gainvalue is stored. After the first gain adjustment, the gain is switchedback to the original level the AGC had before the alternative frequencywas checked. At the beginning of the next time slot, or of any followingtime slot, the gain value that corresponds to the alternative frequencychannel is written to the receiver's AGC, and the receiver's gain isinstantaneously switched to said gain value. Said time slot is then usedfor determining the signal strength at said alternative frequency, andfor detecting which program is transmitted at the alternative frequency.During the second time slot, the rather long settling time is notrequired anymore. Since the AGC is switched, the gain control loop isswitched off, the AGC can not become unstable. The whole second timeslot is available for analysing the broadcast signal at said alternativefrequency. Even in case the automatic gain control of a DRM receiverreacts rather slowly, it becomes possible the track the signal strengthsof a variety of alternative frequencies.

Further preferably, said broadcast signal received at said presentfrequency is correlated with said broadcast signal received at saidalternative frequency. In case the audio programs transmitted at thepresent frequency and at the alternative frequency are identical, a goodcorrelation of the bit streams received at said two frequencies isobtained. In case different programs are transmitted, there is nocorrelation at all. Correlating said two bit streams leads to anunambiguous result.

According to another preferred embodiment of the invention, both thebroadcast signal received at said present frequency and the broadcastsignal received at said alternative frequency are FM signals. Also FMstations often transmit their radio programs via a set of differentfrequencies.

Preferably, from a RDS signal component of the broadcast signal receivedat the alternative frequency, a PI code of the broadcast signal at thealternative frequency is derived, and said PI code of the alternativefrequency is compared with the PI code of the present frequency. A FMbroadcast signal comprises a RDS signal component with a PI codeindicating the respective program or the respective station that istransmitted via said FM signal. Comparing the PI codes of thealternative frequency with the present frequency's PI code is theeasiest way to determine whether both programs are identical or not.Other methods are also possible.

Preferably, the second gain value is set to a predefined constant. Bychoosing a constant that is sufficiently low, saturation of the A/Dconverter can be avoided.

According to a preferred embodiment of the invention, the second gainvalue is determined by reducing the present gain value by a predefinedconstant. For example, the second gain value might be obtained byreducing the present gain value by 40 dB. Alternatively, the gain isswitched to a predetermined value. Even in case the signal received atthe alternative frequency is rather strong, overflow of the controlcircuit's IF A/D converter is avoided, whereby the signal at the IF A/Dconverter's input is still sufficiently high to determine the signalstrength. In case of a weak signal received at the alternativefrequency, the reduced gain will decrease the amplitude of the A/Dconverter's input signal. This is not a disadvantage, though, because incase the signal strength at an alternative frequency is not considerablyhigher than the signal strength at the present frequency, saidalternative frequency doesn't have to be considered anyway.Alternatively, it is possible to iteratively reduce the present gainvalue until there is no overflow anymore, for example in two gain stepswith a step width of 40 dB each.

Preferably, for each of a set of alternative frequencies, acorresponding gain value adapted to the signal strength of the broadcastsignal at said alternative frequency is stored. Further preferably, thegain values corresponding to the various alternative frequencies may bestored in a table maintained at the gain control circuitry. Said tablecontains, for each of said frequencies, the most recent gain value.Whenever a certain alternative frequency is monitored, the gain isinstantaneously switched to the respective gain value obtained from saidtable. This gain value has the right order of magnitude, and therefore,the evaluation and analysis of the broadcast signal received at therespective alternative frequency can start immediately. In a shortperiod of time, a lot of different alternative frequencies can bemonitored, and the frequency with the best signal quality can beselected.

The invention is explained taking reference to the following remarks:

The invention is related to the test of alternative frequencies forbroadcast systems like FM or DRM. The invention describes a strategy inreceivers with a digital controlled AGC that allows quick checks ofalternative frequencies.

In a receiver with a long delay in the AGC loop, for example in an AGCloop that is controlled by the IF signal and that controls the RFamplifier of the receiver, a fast time constant of the loop can not beimplemented. A fast time constant would lead to an instable AGC loop.

A DRM receiver checks the alternative frequency by switching from thecurrent frequency to a possible alternative frequency and checks thedata by a correlation of the received data with the expected data. Thetime for switching the PLL to the alternative frequency, setting the AGCgain, reading the data from the alternative frequency, switching backthe PLL to the original frequency and setting the AGC to the originallevel is very short. In case the AGC can only be realised with a longtime constant, as it is the case in the receiver above, the programmableAGC can be used to solve the problem.

The receiver performs the DRM AF check in two steps: In a first step,during the first Static Data symbol, the receiver stores the AGC controlvoltage (corresponding to a certain fieldstrength) of the currentstation. Then the receiver switches to the alternative frequency. TheAGC needs some time for settling to the correct gain, for example about10-20 ms. The receiver stores the final AGC control value thatcorresponds to the fieldstrength of the station at the alternativefrequency. Then the receiver switches back to the originally receivedfrequency. The AGC control voltage is switched back to the stored valueof the received station.

In a second step, during the next static data symbol, the receiverswitches back to the same alternative frequency. The AGC sets thepreviously (in step 1) determined AGC control value, so that the AGCgain is in the correct range. The slow AGC does not need any settlingany more, so the receiver can directly check the data of the alternativefrequency.

In order to avoid an IF ADC overflow, the AGC gain should slightly bereduced when switching to the AF the second time. A low IF ADC inputrange can be handled by the digital signal processing, a too high IF ADCinput signal leads to an IF ADC saturation. The AF can not be checked.

A similar system could be used to check alternative frequencies in FMbroadcast. The receiver knows the fieldstrength of the current station.The receiver only switches to the AF in case the fieldstrength of the AFis higher. In case of a slow AGC, the AGC switches to a lower gain thanthe gain at the current station to check the alternative frequency. Forexample, the AGC gain is reduced by 40 dB or more (depending on thecurrent fieldstrength) during the switching to the alternativefrequency. In a first step, the receiver only measures the fieldstrengthof the AF: The AGC gain is reduced by about 40 dB, so that there is alow risk that the IF ADC is in saturation. The receiver measures thefieldstrength of the AF by combining the knowledge of the AGC gain (thatwas set manually) and the measurement of the amplitude of the FM inputsignal in the digital receiver. In a more advanced receiver, thereceiver stores the results of the measurements of the fieldstrengths ofthe alternative frequencies and uses this information to set the properAGC gain before the actual fieldstrength measurement is done. In asecond step, the receiver properly sets the AGC gain and starts with thedecoding of the RDS data to check the stations PI code

According to the present invention, inter alia a method is proposed thatis required to perform a DRM AF check in a receiver with a slow AGCloop.

In the following, the invention will be described in more detail takingreference to the accompanying figures on the basis of preferredembodiments of the invention.

FIG. 1 is a receiver with a gain control loop having a rather large timeconstant;

FIG. 2 is the structure of a DRM signal.

In a receiver with a long delay in the automatic gain control loop, afast attack time cannot be implemented. An example for such a receiveris shown in FIG. 1. The invention is not limited to the gain controlcircuit shown in FIG. 1. It can be applied to any gain control circuitto which the time constant for adjusting the gain is rather high.

A RF signal 2 received via an antenna 1 is provided to a RF amplifier 3.An amplified RF signal 4 is downconverted, by a oscillator 5, to anintermediate frequency range. The oscillator 5 multiplies the amplifiedRF signal 4 with a signal of constant frequency f0. The downconvertedsignal 6 is provided to an intermediate frequency filter 7, and thefiltered signal 8 is amplified by an IF amplifier 9. The analogue IFsignal 10 is converted, by an A/D converter 11, to a digital IF signal12. The digital IF signal 12 serves as a starting point for furthersignal processing and signal decoding. Besides that, the digital signal12 is provided to an automatic gain control unit 13, and there, thesignal strength of the digital IF signal 12 is compared to a referencemagnitude. The automatic gain control unit 13 generates a digital gaincontrol signal 14 that is converted, by a D/A converter 15, into ananalogue gain control signal 16. The analogue gain control signal 16 isprovided to the RF amplifier 3, and the gain of said amplifier is variedin accordance with the analogue gain control signal 16.

In the AGC loop shown in FIG. 1, the RF amplifier's gain is controlledby the control signal, while the IF amplifier's output signal is usedfor deriving the gain control signal. Between the RF amplifier 3 and theIF amplifier 9, the group delay of the intermediate frequency filter 7causes a time lag between the RF amplifier's output and the gain controlsignal. This additional delay limits the attack time of the closed-loopcontrol circuit, and therefore, it takes a long time for the closed-loopcontrol circuit to adjust to a different signal strength.

Many broadcast stations transmit a certain program via a set ofdifferent frequencies. On part of the receiver, the signal strengths ofthe broadcast signals received at different frequencies may be compared.Then, the receiver may select a reception frequency where the receptionconditions are convincing. In order to monitor the broadcast signals atvarious alternative frequencies, the receiver has to switch from thecurrent frequency to a possible alternative frequency, and back to thecurrent frequency, whereby the signal reception at said currentfrequency must not be interrupted. Therefore, the time for switching thereceiver's PLL (Phase Locked Loop) to the alternative frequency, forsetting the AGC gain to the new gain value, for reading the data fromthe signal received at the alternative frequency, for switching back thePLL to the original frequency, and for readjusting the AGC gain to theoriginal value is very short. The automatic gain control circuit shownin FIG. 1 cannot be switched to a fast mode, because the AGC loop wouldbecome instable.

In the following discussion, it is assumed that both the broadcastsignal received at the current frequency and the broadcast signalsreceived at alternative frequencies are signals according to the DRM(Digital Radio Mondiale) standard. In FIG. 2, the structure of a DRMsignal is shown as a function of time. The time intervals 17, 19, 21have an order of magnitude of about 1.2 sec. During these timeintervals, the data symbols that carry the bits and bytes of therespective radio program's data stream are transmitted. Each of the longtime intervals 17, 19, 21 is followed by a short time slot 18, 20, 22,and during these short time slots of approximately 18.66 to 48 ms,static data symbols are transmitted. The reception of static datasymbols during the time slots 18, 20, 22 does not cause any audibledistortions for the listener.

The time slots 18, 20, 22 can therefore be used for monitoringalternative frequencies. Monitoring an alternative frequency requiresthe steps of changing the receiver's PLL to the alternative frequency,settling the gain of the receiver to the signal strength at saidalternative frequency, evaluating the received data, and switching backto the current frequency. Because of the rather large settling time ofthe automatic gain control circuit, it is impossible to perform allthese tasks during a single one of the time intervals 18, 20, 22.

According to the invention, the monitoring of a broadcast signal at analternative frequency is performed as follows: during a first one of thetime slots for static data symbol reception, for example during the timeslot 18, the receiver's PLL is switched to the alternative frequency,and the control loop of the gain control circuit starts to adjust thegain to the signal strength of the broadcast signal received at thealternative frequency. At the end of the time interval 18, thereceiver's gain has settled to the signal strength of the alternativesignal. The appropriate gain value corresponding to the alternativefrequency is stored. For example, the respective gain value might bestored in a register of a programmable AGC circuit. At the end of thetime slot 18, the PLL is switched back from the alternative frequency tothe current frequency, the gain value corresponding to the currentfrequency is restored, and during the following time interval 19,reception of the data stream transmitted at the current frequency iscontinued.

At the beginning of the time slot 20, the received frequency is againswitched to the alternative frequency, and the gain value correspondingto said alternative frequency, which has been determined during the timeslot 18, is written to the receiver's gain control unit. In order tomake sure that an overflow of the A/D converter is avoided, the storedAGC gain should be slightly reduced before it is written to thereceiver's gain control unit. While a low input range of the A/Dconverter can be handled by the digital signal processing, an ADC inputsignal that is too high leads to a saturation of the A/D converter, andit becomes impossible to check the broadcast signal at the alternativefrequency. The AGC's “settling time”, which might have an order ofmagnitude of 10 to 20 ms, is replaced by a “setting time” of less than 1msec. At the beginning of the time slot 20, the gain value isinstantaneously switched to an appropriate value, and though the AGC'stime constant is rather high, decoding of the broadcast signaltransmitted at the alternative frequency can start immediately.

Next, it has to be checked whether the radio program transmitted at thealternative frequency is identical to the radio program of the currentfrequency. For this purpose, the bit stream received during the timeslot 20, after the received frequency has been set to the alternativefrequency, is correlated to the bit stream corresponding to the currentfrequency's radio program, which is already known at the beginning oftime slot 20. In case the radio programs transmitted at the currentfrequency and at the alternative frequency are identical, it might makesense to switch to the alternative frequency. Before doing that, it hasto be checked if the signal strength at the alternative frequency isindeed higher than the signal strength at the present frequency. Thesignal strengths can e.g. be compared by relating the gain value of thealternative frequency to the gain value of the present frequency. Incase the signal strength at said alternative frequency exceeds thesignal strength at the current frequency by a predefined amount, it canbe concluded that a change of the PLL's frequency would considerablyimprove the reception conditions. In this case, the received frequencyis switched over to the alternative frequency, and the reception of thecurrent audio program is continued at the alternative frequency.

The two tasks of correlating the bit streams transmitted at the presentfrequency and at the alternative frequency and of comparing the signalstrengths may be performed in arbitrary order, and they might as well beperformed in parallel. Both tasks are performed during the time slot 20,and at the end of time slot 20, it is clear whether frequency switchingshould be performed or not.

The invention might as well be used for monitoring a whole set ofalternative frequencies. There do exist radio stations that broadcasttheir radio programs at a multitude of different frequencies. For eachone of said alternative frequencies, a separate gain value might bedetermined and stored in a dedicated register. As soon as these gainvalues are available, it becomes possible to quickly switch from onefrequency to another frequency without been restricted by the AGC'ssettling time. The signal strengths and the programs at severalalternative frequencies can be monitored during the reception of thecurrent frequency's broadcast signal. Then, the frequency channel withthe best reception conditions can be selected.

The invention can also be used for checking alternative frequencies whenreceiving a FM broadcast signal. FM signals do not comprise a time slotstructure as shown in FIG. 2. When switching the received frequency toan alternative frequency, the gain level of the automatic gain controlis set to a predefined gain value that corresponds to the expectedsignal strength at said alternative frequency. For example, for eachalternative frequency of a set of alternative frequencies, acorresponding gain value might be stored, e.g. in a table maintained inthe gain control circuitry. Whenever a certain alternative frequency ismonitored, the corresponding gain value is written to the automatic gaincontrol. The time period for adjusting the receiver's gain to thealternative frequency signal is considerably reduced.

Another strategy is to use the gain value at the present frequency as astarting point, and to decrease said gain value by a predefined amount.The dynamic range of a FM receiver's input signal covers about 120 dB,and therefore, the AGC gain at the present frequency might for examplebe reduced by 40 dB or more (depending on the current signal strength)when switching to an alternative frequency. In case the signal strengthof the FM signal at said alternative frequency is considerably higherthan the signal strength at the present frequency, there is a low riskthat saturation of the control circuit's A/D converter occurs, andprocessing of the received can start immediately. In case the FM signalreceived at the alternative frequency is weak, the gain level (which hasbeen reduced by 40 dB) might be to small. This doesn't matter, though,because a FM signal with a low signal strength is not worth to beanalysed anyway. Another strategy is to iteratively reduce the gainvalue, for example in steps of 20 or 40 dB, when checking a FM signal atan alternative frequency.

Next, the signal strength at the alternative frequency is determined.The receiver determines the signal strength of the FM signal received atthe alternative frequency by combining the knowledge of the AGC gain andthe measurement of the FM signal's amplitude. Then, in a second step,the receiver starts decoding the FM data. In particular, the PI (ProgramInformation) of the radio program transmitted at the alternativefrequency can be obtained by decoding the RDS (Radio Data System) signalcomponent of the FM signal. By comparing the PI code of the alternativefrequency's radio program with the PI code of the current radio program,it is possible to determine whether the programs are identical or not.In case the signal strength at the alternative frequency is considerablyhigher than the signal strength at the present frequency, and in casethe programs are identical, it makes sense to switch to the alternativefrequency and to continue receiving said radio program at thealternative frequency. The listener might not even notice that thereceived frequency has been changed.

LIST OF REFERENCE SYMBOLS

-   1 antenna-   2 RF signal-   3 RF amplifier-   4 amplified RF signal-   5 oscillator-   6 downconverted signal-   7 intermediate frequency filter-   8 filtered signal-   9 IF amplifier-   10 analogue IF signal-   11 A/D converter-   12 digital IF signal-   13 automatic gain control unit-   14 digital gain control signal-   15 D/A converter-   16 analogue gain control signal-   17, 19, 21 time intervals-   18, 20, 22 time slots

1. A method for monitoring DRM broadcast signals at alternativefrequencies during reception of a DRM broadcast signal at a presentfrequency, said DRM broadcast signals including static data symbolsduring time slots and radio program data during time intervals, saidtime slots and said time intervals being arranged in a sequence, inwhich one time slot is followed by one time interval and vice versa,said method comprising: receiving radio program data at a presentfrequency during a first time interval with a receiver gain of a presentgain value; switching to an alternative frequency during a first timeslot, wherein during said first time slot said receiver's gain settlesto a second gain value; switching to said present frequency during asecond time interval; receiving further radio program data at saidpresent frequency during said second time interval with a receiver gainof said present gain value; switching to said alternative frequencyduring a second time slot to check a broadcast signal; instantaneouslyswitching the receiver's gain from said present gain value to saidsecond gain value; and checking said broadcast signal at saidalternative frequency during said second time slot.
 2. The methodaccording to claim 1, further comprising: determining whether theprogram transmitted via the broadcast signal at said alternativefrequency is the same as the program transmitted via the broadcastsignal at the present frequency.
 3. The method according to claim 1,further comprising: comparing the signal strength of the broadcastsignal received at the alternative frequency to the signal strength ofthe broadcast signal received at the present frequency.
 4. The methodaccording to claim 1, wherein when the signal strength of the broadcastsignal at the alternative frequency surpasses the signal strength of thesignal at the present frequency by a predefined amount, and when theprograms transmitted at both frequencies are identical, the receivedfrequency is switched from the present frequency to the alternativefrequency.
 5. The method according to claim 1, further comprising:correlating said broadcast signal received at said present frequency andsaid broadcast signal received at said alternative frequency.
 6. Themethod according to claim 1, wherein the second gain value is set to apredefined constant.
 7. The method according to claim 1, wherein thesecond gain value is determined by reducing the present gain value by apredefined constant.
 8. The method according to claim 1, wherein thesecond gain value is determined by iteratively reducing the present gainvalue, whereby in each step, the present gain value is reduced by apredefined constant.
 9. The method according to claim 1, furthercomprising: storing a corresponding gain value adapted to the signalstrength of the broadcast signal at said alternative frequency for eachset of alternative frequencies.
 10. A program stored on a computerreadable medium, for causing a computer, when said program is executedon a computer or digital signal processor, to perform the method asdefined in claim
 1. 11. The method according to claim 1, wherein thesecond gain value is determined by iteratively modifying a predeterminedgain value.
 12. A receiver for receiving DRM broadcast signals includingstatic data symbols during time slots and radio program data during timeintervals, said time slots and said time intervals being arranged in asequence, in which one time slot is followed by one time interval andvice versa, said receiver being configured: to receive radio programdata at a present frequency during a first time interval with a receivergain of a present gain value; to switch to an alternative frequencyduring a first time slot, wherein during said first time slot saidreceiver's gain settles to a second gain value; to switch to saidpresent frequency during a second time interval; to receive furtherradio program data at said present frequency during said second timeinterval with a receiver gain of said present gain value; to switch tosaid alternative frequency during a second time slot to check abroadcast signal; and said receiver comprising: a gain control unit,wherein said gain control unit comprises: gain switching means forinstantaneously switching the receiver's gain from said present gainvalue to said second gain value when said broadcast signal at saidalternative frequency is checked.
 13. The receiver according to claim12, further comprising: comparator means adapted for comparing thesignal strength of the broadcast signal received at the alternativefrequency to the signal strength of the broadcast signal received at thepresent frequency.
 14. The receiver according to claim 12, furthercomprising: frequency switching means adapted for switching the receivedfrequency from the present frequency to the alternative frequency whenthe signal strength of the broadcast signal at the alternative frequencysurpasses the signal strength of the signal at the present frequency,and when the programs transmitted at both frequencies are identical. 15.The receiver according to claim 12, further comprising: a correlatoradapted for correlating said broadcast signal received at said presentfrequency and said broadcast signal received at said alternativefrequency.
 16. The receiver according to claim 12, further comprising:storage means adapted for storing, for each of a set of alternativefrequencies, a corresponding gain value adapted to the signal strengthof the broadcast signal at said alternative frequency.